1. Field of Invention
The present invention relates to a method for controlling the elastic modulus of piezoelectric substances with which the elastic modulus of piezoelectric substances can be changed to a great degree based on a simple method. The present invention also relates to a piezoelectric device having a controllable effective elastic modulus.
2. Description of Related Art
In general, a piezoelectric substance generates an electromotive force when an external mechanical force is applied to it. Such an effect is called the combined mechanical-electrical effect. The generated electromotive force is combined with the applied external force which originally causes a deformation, and causes a further deformation of the substance. This effect is called the combined electrical-mechanical effect.
In this case, the generated electromotive force which causes the additional deformation counteracts the applied mechanical force. As a result, the piezoelectric substance appears to be hardened. The electromotive force generated can be measured in the form of a voltage by installing electrodes on the piezoelectricity generating surface of the piezoelectric substance.
The magnitude of the electrical-mechanical reaction, which represents the apparent rigidity of the piezoelectric substance, changes as the electrodes are shorted and opened.
However, the elastic modulus of the piezoelectric substance changes by at most a few percent by simply shorting and/or opening the electrodes installed on the piezoelectricity generating surface of the piezoelectric substance. This is due to the fact that the magnitude of this effect is proportional to the square of the electrical-mechanical coupling coefficient k, and that the value of k is at most about 0.2 in the case of an ordinary piezoelectric substance.
Hence, the amount of change in the elastic modulus of the piezoelectric substance achievable by the conventional method is at most a few percent, and does not exceed the margin of error in the measurement of the elastic modulus of the piezoelectric substance. So far, there has been no way to change the elastic modulus of a piezoelectric substance to any great extent even though it has been known that a piezoelectric substance with large achievable amount of change in the elastic modulus provides a device applicable to various fields.